import duckdb
import narwhals as nw
rel = duckdb.sql('select * from values (1, 4), (1, 2), (2, 3), (2, 4) df(a, b)')
df = nw.from_native(rel)
 df.select(nw.sum_horizontal(1, 'a', 'b'))

InvalidIntoExprError: Expected an object which can be converted into an expression, got <class 'int'>

Hint:
- if you were trying to select a column which does not have a string
  column name, then you should explicitly use `nw.col`.
  For example, `df.select(nw.col(0))` if you have a column named `0`.
- if you were trying to create a new literal column, then you
  should explicitly use `nw.lit`.
  For example, `df.select(nw.lit(0))` if you want to create a new
  column with literal value `0`.